1. Field of the Disclosure
The disclosure relates generally to electrophotograhic (EP) devices, and, more particularly, to compensating misalignment errors in EP devices.
2. Description of the Related Art
Printing devices, such as electrophotographic (EP) devices, are widely used in offices, homes and business enterprises for printing information on media sheets, such as papers, transparencies, envelopes, and the like. EP devices such as EP printers operate by generating an image pattern of information to be printed, and subsequently transferring toner particles onto a media sheet based on the generated image pattern to produce an image of the information on the media sheet. The image of the information transferred onto the media sheet may be adhered onto the media sheet by application of heat and pressure.
The information transferred onto the media sheet may be black-and-white information or colored information. Currently, various multicolor EP printers are available that print colored images of the information onto the media sheet. The multicolor EP printers include four toner cartridges for carrying four different colored toners, namely, cyan, magenta, yellow and black (which colors may hereinafter be referred to as “CMYB”). The four colors may be combined into varying proportions to generate a full spectrum of colors. Typically, each of the four colors may be deposited on the media sheet to form sub-images. In a single-pass multicolor EP printer, sub-images derived from four different colors must lie precisely aligned with respect to each other.
A single pass EP printer includes an imaging assembly for printing information onto the media sheet. The imaging assembly includes a laser beam source, a photoconductive member such as a photoconductive drum, and a toner cartridge, for each of the four colors. Moreover, the imaging assembly includes a fuser. The photoconductive member is typically homogenously charged. Based on the information to be printed on the media sheet, each of the laser beam sources focus a light beam onto the photoconductive drum to create a charge pattern on the photoconductive drum. Toner particles are electrostatically attracted to the charge pattern to form a latent image of the information, which may then be transferred on to the media sheet to form an image onto the media sheet.
The image formed on the media sheet includes unfused toner particles that need to be fused, thereby forcing the unfused toner particles to adhere to the media sheet. The media sheet having unfused toner particles deposited thereon may be advanced towards the fuser for fusing the toner particles onto the media sheet. The fuser may typically include a heater device for applying heat onto the unfused toner particles for permanently fixing the image onto the media sheet. The high temperature melts the toner particles and allows the toner particles to adhere to the media sheet. However, the high temperature produced by the fuser may also result in thermally induced expansion and contraction of optical components, such as lens elements in the EP printer, thereby changing refractive indices thereof. The variation in refractive indices of the optical components may cause process and/or scan direction shifts in a scan path of the laser beam, relative to an original scan path of the laser beam. The process and/or direction shifts have the effect of causing misalignment errors in the single-pass multicolor EP printer. Further, due to heating of the optical components, the laser beam's time-of-flight also gets changed, thereby affecting color-to-color registration in the single-pass multicolor EP printer.
Currently available EP printers employ sensors for measuring the process/scan direction shifts in the path of the laser beam. The sensors detect a Start-of-scan (SOS) time and an End-of-scan (EOS) time of the laser beam for determining time-of-flight of the laser beam. The EP printer is calibrated to shift the color-to-color registration (hereinafter referred to as registration correction) as a function of the time-of-flight of the laser beam for adjusting alignment thereof. However, this method of calibrating the multicolor EP printer may still cause errors in determining registration correction since the method is based on the assumption that the heat produced in the EP printer affects only the optical components. Typically, the heat produced in the EP printer also affects various other components, such as distortion of a printhead housing, and/or mounting location of the sensors that are used to generate the SOS time and the EOS time. The distortion of the printhead housing may be non-linear, thereby affecting the lens' position and subsequently altering the position of the SOS and the EOS sensors. The alteration of the position of the SOS and the EOS sensors results in misregistration errors in the EP printer.
Alternative methods available in prior art for calibrating the EP printer include measuring registration correction as a function of temperature of a laser printhead. Specifically, the temperature of the laser printhead is sensed using a thermistor, located within, or near the laser printhead. The registration correction is characterized as a function of thermistor readings, and then a response is assumed to be constant over a population of EP printers. Using this method, the registration correction is determined as a function of the change in temperature, using a characterized relationship, and assuming a linear response with temperature. However, this assumption of the linear response with temperature may further cause errors in determination of registration corrections, since the temperature rise sensed by the thermistor may be due to multiple input sources that may affect the registration correction in various ways. Moreover, registration response to heat may also depend on the rate of heat entering or leaving the EP printer.
Based on the foregoing, there is a need for compensating misalignment errors in EP devices by appropriate means taking into consideration various heat generating sources in the EP devices.